Correlation studies and theoretical models suggest the major plate driving forces, slab pull and ridge push, are closely linked to seafloor age at subduction zones (=transit time and hereafter denoted &tgr;). Based on this fact it has been proposed that seafloor spreading rate fluctuations should be stably damped because a speedup should lead to rejuvenation of &tgr; and hence a weakening of driving forces. If true, such stability could imply the first-order cycle in Pacific spreading rates during the Mesozoic-Cenozoic (''the pulse of the Earth'') was externally forced by mantle dynamics. However we show that a crucial assumption in this stability argument is invalid: increasing spreading rates do not necessarily cause transit time to decrese. The transit time dynamics of seafloor are complex; simple plate models suggest that, rather than stabilizing spreading rate, &tgr;-dependent plate driving forces may be destabilizing leading to self-sustained and aperiodic oscillations. These results raise questions about the cause of spreading rate cycles. Most importantly, are the plates themselves, through age dynamics, actively contributing to ''pulse of the Earth''? ¿ American Geophysical Union 1994